This invention relates to a process for removing suspended solids from waste water streams, particularly primary and secondary effluents from municipal sources or industrial processes such as paper mills and vegetable canning operations. It is more particularly directed to the filtration of fine suspended solids from effluents resulting from paper processing, notably white water effluents and secondary effluents from such processes.
Filtration has long been a major method of removing solids from liquid streams. In waste water treatment, the removal of suspended solids is a difficult task, and several filtration means have been employed to deal with these removal problems. Sand filters and multi-media filters have been widely used for some applications. Work in the field has shown these filters to be generally effective in removing suspended solids, but only from streams of low suspended solids content and at relatively low flow rates through the filters. In general, solids concentration of the stream to the filter must be below about 200 milligrams/liter. Suspended solids concentration values above this level tend to lead to filtration bed clogging and high pressure drop across the bed. Thus, the application of sand or multi-media filters is limited to certain effluent streams. The restriction of low flow rates also precludes their use on high volume municipal or industrial waste water streams.
Efforts to use other materials, such as solid or foam polymeric materials, to remove suspended solids has been limited to use of discontinuous particles. Here again deep beds of solid or foam polymeric particles were used and removal of suspended solids was mainly accomplished by physical entrapment of the suspended solids on the polymeric particles at some point in the deep bed. Plugging of the bed, pressure drops, and/or low flow rates also characterized these efforts. In addition, there is the problem of foam particles being thrown out of the bed during backwashing. One attempt to overcome this problem is described in U.S. Pat. No. 4,162,216 which uses mechanical agitation to remove the suspended solids from the foam particles.
In the past, filtration using layers of polymeric foam, such as polyurethane foam, has been limited primarily to the separation of oil from water. The oleophilic nature of the foam and the tendency of oil to coalesce on the foam have made polyurethane foams especially useful for this type of separation process. Such processes are taught, for example, in Grutsch, U.S. Pat. No. 3,608,727, Johnson et al, U.S. Pat. No. 3,617,551, and De Young, U.S. Pat. No. 3,888,766. Teitsma, U.S. Pat. No. 3,334,042 teaches that carbon particles can be agglomerated with liquid hydrocarbons and then separated out from a water phase using various polymer foams. And in a paper entitled "A Filter-Coalescer Device For Oil-Water Separation" prepared for presentation at the Sixth Annual Offshore Technology Conference in Houston, Tex., in May, 1974, and authored by Arye Gollan and Daniel H. Fruman, there is disclosed a system for separating oil from water using polyurethane foam. The paper notes that, along with oil retention, there is some accompanying solids retention. It is clear, however, from the disclosures of U.S. Pat. No. 3,334,042 and the Gollan and Fruman paper that the separation using polyurethane foams is a liquid-liquid separation, not a liquid-solid separation.